Topiary trees have leafy, thick foliage that gardeners can form or train to create everything from a simple ball and three-dimensional objects to human or animal sculptures. Although you can prune just about any tree, some outdoor trees are better than others for enduring this type of intense pruning and shaping. These long-living evergreen trees grow somewhat slowly and don't need as much pruning and trimming as other trees, which helps for creating topiary art. Their thick form also contributes to why they're commonly used as their leaves can be closely clipped for forming solid and sharp-edged shapes.